1. Field of the Invention
The present invention relates to a mobile communication system adopting CDMA (DS-CDMA) for performing spread spectrum multiple access. More particularly, the present invention relates to a receiving process method for canceling interference due to multipath when high speed data transmission is performed for down-link transmission from a base station in cellular communication using DS-CDMA.
In addition, the present invention relates to a receiving apparatus which can remove multipath interference according to the receiving process method.
2. Description of the Related Art
Wide band DS-CDMA (W-CDMA) has been adopted as a wireless access method in the next generation mobile communication method IMT-2000 (International Mobile Telecommunication 2000) The maximum information transmission speed in IMT-2000 is 144 kbps in a mobile environment, 384 kbps in a walking environment and 2 Mbps in a quasi-still environment. Thus, it is predicted that multimedia services in addition to voice services will be provided in the next generation mobile communication system. On the other hand, when considering rapid popularization of the Internet, multiplicity of information, enlarging capacity and developments of the next generation internet in recent years, it becomes necessary to develop a wireless access method for realizing information transmission speed exceeding 2 Mbps in mobile communication. Especially in down-link transmission from the base station, it is considered that high-speed and large capacity traffic due to download of images, files, video and the like from databases and web sites will increase. Therefore, a high speed packet transmission technology suitable for such data traffic becomes necessary. Against this backdrop, ultrahigh-speed packet transmission by extending the W-CDMA wireless interface is being studied for realizing high-speed packet transmission exceeding 2 Mbps. For example, application of adaptive modulation/demodulation and error correction (channel coding) and ARQ (Automatic Repeat reQuest) based on adaptive wireless link control (link adaptation) is studied. The adaptive modulation/demodulation and error correction based on the link adaptation are methods for switching modulation level (number of bits in one symbol), SF (spreading factor), multi-code multiplexing number, and coding ratio of error correction according to propagation environment of each user in order to perform high speed data transmission effectively. For example, as the propagation environment for a user becomes better, maximum throughput of the mobile communication system can be increased by switching modulation methods of W-CDMA from QPSK to more effective multilevel modulation, that is, 8 PSK, 16 QAM, 64 QAM modulation. For example, when SF=4, multi-code number is 3, error correction coding ratio is ½, and 64 QAM is used for data modulation, 8.5 Mbps ultrahigh-speed data transmission is theoretically possible by using a W-CDMA wireless interface of chip rate 3.84 Mcps.
As mentioned above, 8.5 Mbps ultrahigh-speed data transmission is theoretically possible by increasing the modulation level, decreasing SF (increasing multi-code number), and increasing coding ratio of error correction. However, increasing the modulation level leads to increasing required desired wave signal power to interference power ratio (SIR) which is necessary for satisfying the same receiving quality (bit error rate).
In addition, when applying the adaptive modulation/demodulation and error correction to an actual mobile communication environment, tolerance to multipath fading (frequency selective fading) becomes important. For example, orthogonalization between users (between code channels) is possible in the same transmission path in a down-link in the W-CDMA. However, degradation of transmission quality occurs due to interference between multipaths under multipath environment.
Generally in DS-CDMA, this multipath interference can be suppressed to 1/SF of received signal power on average for each code channel like general multi-user interference. However, for performing ultrahigh-speed data transmission of 8.5 Mbps by using W-CDMA wireless interface having chip rate 3.84 Mcps, it is necessary to decrease SF near to 1 and increase the multi-code number for increasing data rate. In this case, degradation of received SIR due to multipath interference becomes very large. As a result, even when other user does not exist and even when background noise such as thermal noise is small, an area for realizing high-speed packet transmission of multilevel modulation, low SF and high coding ratio is limited to an area very close to a base station where there is no multipath interference so that average throughput of the system deteriorates.